1. Field of the Invention
The present invention relates to a variable valve lift device used for a direct drive-style valve driving system driving directly an intake or exhaust valve (hereafter, referred as a valve) using a cam when the valve of an internal combustion engine (hereafter, referred as an engine) is opened and closed. The variable valve lift device (hereafter, referred as a VVL device) changes a length of a tappet in an axial direction to adjust a valve lift of the valve.
2. Description of the Prior Art
With a conventional rocker arm-style engine, in order to adjust the valve lift of the valve responsive to any operational status of the engine, cam-profiles arranged on a camshaft, each profile corresponding to a required valve lift, and a switching mechanism is arranged in the rocker arm. With such a constructed engine, it is necessary to arrange the plurality of cam profiles on the camshaft and accordingly to increase the cost of manufacture. It further runs counter to a request of reduction in weight.
On the other hand, in recent years, the direct drive-style engine driving directly the valve using a cam without using the rocker arm provides with the VVL device adjusting the valve lift of the valve responsive to any operational status of the engine. A known VVL device includes a tappet case arranged between the cam and the valve and a variable valve lift system built into the tappet case. Construction regarding a general direct drive-style engine and the conventional VVL device will be described in detail hereafter.
FIG. 1 is a diagrammatic sketch of direct drive-style valve driving system in the engine. FIG. 2 is a view taken in the direction of arrow Axe2x80x94A in FIG. 1 and showing a cam on a camshaft in the valve driving system. FIG. 3 is a front view of a cam-profile of a cam shown in FIG. 2. Here, only an intake valve driving system of the intake and exhaust valve driving systems is indicated. Since the exhaust valve driving system has the same construction as the intake valve driving system, the exhaust valve driving system is also operated by the same action as the intake valve driving system. Therefore, further description will be omitted. Moreover, assume that a cylinder shown in the drawing is arranged in a vertical direction.
In the drawings, reference numerals 1, 2, 3 and 4 denote cylinders in a four-cylinder engine. Pistons 5, 6, 7 and 8 are arranged in the respective cylinders 1, 2, 3 and 4, each reciprocating in an axial direction of each cylinder. Reciprocal movements of the pistons 5, 6, 7 and 8 are converted into rotational movements and transferred to a crankshaft 13. Two valve seats 14, 15, 16 and 17 per cylinder are arranged at an upper section (cylinder head) of the cylinders 1, 2, 3 and 4, respectively. Intake valves 18, 19, 20 and 21 are arranged at the valve seats 14, 15, 16 and 17, respectively. Rotational movements of intake cams 26, 27, 28 and 29 are transferred to the intake valves 18, 19, 20 and 21 byway of a VVL devices 22, 23, 24 and 25. The intake cams 26, 27, 28 and 29 are arranged on an intake camshaft 30. The intake camshaft 30 can be rotated in a direction of arrow B in FIG. 2 due to a rotational driving force of the crankshaft transferred to the intake camshaft 30 via a pulley 31, a driving force transferable member 32 such as timing belts, and a pulley 33.
Here, since all the intake cams 26, 27, 28 and 29 have the same construction, the intake cam 26 will be explained as a representative example. The intake cam 26 shown in FIG. 3 includes a base-circle section 26a having a circular-shape in cross section, a lift-curve section 26b protruded from the base-circle section 26a and two ramp sections 26c and 26d connecting smoothly the base-circle section 26a to the lift-curve section 26b and vice versa. Another intake cams 27, 28 and 29 have the construction above as in the case of the intake cam 26.
The lift-curve section 27b of the intake cam 27 and the lift-curve section 28b of the intake cam 28 are shifted plus or minus 90 degrees with respect to the lift-curve section 26b of the intake cam 26 in an outer periphery of the intake camshaft 30 as shown in FIG. 2. The lift-curve section 29b of the residual intake cam 29 is shifted approximately 180 degrees with respect to the lift-curve section 26b of the intake cam 26 in an outer periphery of the intake camshaft 30.
Here, since all the VVL devices 22, 23, 24 and 25 have the same construction, the VVL device 22 will be explained as a representative example. The conventional VVL device 22 has a construction as disclosed in German Patent Gazette DT1958627. The conventional VVL device 22 includes a tappet case 34 having an upper section with a cam contact section 34a making contact with a cam face of the intake cam 26. The device 22 includes a hydraulic cylinder (not shown) arranged in the tappet case 34 to select a high-lift mode extending the length of the tappet in the axial direction and a low-lift mode shrinking it.
A lower section of the VVL device 22 makes contact with an upper section of a valve stem 35. The intake valve 18 is mounted on a lower section of the valve stem 35. A valve spring (not shown) is arranged between the valve stem 35 and the cylinder 1 and biases upwardly the valve stem 35 in the axial direction to press the intake valve 18 against the valve seat 14 to close it.
An operation of the VVL device 22 will be explained hereafter.
First, just after the engine is started, a hydraulic pressure supplied from an oil pump (not shown) to the VVL device 22 does not yet rise to adequate levels and the hydraulic cylinder (not shown) in the VVL device 22 is not extended. Therefore, the hydraulic cylinder (not shown) is so set as to select the low-lift mode. With the low-lift mode, when the intake cam 26 rotates in the direction of arrow B in FIG. 2, the cam contact section 34a of the tappet case 34 makes contact with the intake cam 26 to run from the base-circle section 26a to the lift-curve section 26b via the ramp section 26c. However, a downward displacement of the cam contact section 34a in the axial direction is not yet increased. Therefore, the tappet case 34 and the valve stem 35 do not move downwardly in the axial direction. When the intake cam 26 further rotates, the cam contact section 34a of the tappet case 34 makes contact with the intake cam 26 to run from the ramp section 26c to a middle of the lift-curve section 26b. At this time, the downward displacement of the cam contact section 34a in the axial direction is increased. Therefore, the tappet case 34 and the valve stem 35 are pressed down against the biasing force of the valve spring (not shown) and the intake valve 18 also is pressed down in the axial direction with respect to the valve seat 14 (low-lift state).
Moreover, when the engine is driven usually, the hydraulic pressure supplied from an oil pump (not shown) to the VVL device 22 rises to adequate levels and the hydraulic cylinder (not shown) in the VVL device 22 is extended. Therefore, the hydraulic cylinder (not shown) is so set as to select the high-lift mode. With the high-lift mode, when the intake cam 26 rotates in the direction B in FIG. 2, the cam contact section 34a of the tappet case 34 makes contact with the intake cam 26 to run from the base-circle section 26a to the lift-curve section 26b in orderly sequence. However, a downward displacement of the cam contact section 34a in the axial direction is not yet increased. Therefore, the tappet case 34 and the valve stem 35 do not move downwardly in the axial direction. When the intake cam 26 further rotates, the cam contact section 34a of the tappet case 34 makes contact with the intake cam 26 to run from the base-circle section 26a to the lift-curve section 26b via the ramp section 26c. At this time, the downward displacement of the cam contact section 34a in the axial direction is increased. The tappet case 34 and the valve stem 35 are therefore pressed down against the biasing force of the valve spring (not shown) in the axial direction in accordance with the cam-profile of the lift-curve section 26b. As a result, the intake valve 18 also is pressed down in the axial direction with respect to the valve seat 14 (high-lift state).
However, the VVL device 22 provides with a variable valve lift system including the hydraulic cylinder (not shown) having oil paths arranged in the tappet case 34. Since the construction of the variable valve lift system is complicated, it results in increasing in mass of the system. Therefore, it becomes useless to reduce inertial mass defined as the maximum merit of the direct drive-style VVL device.
Accordingly, it is an object of the present invention to provide a VVL device which is compact and lightweight and which simplifies its internal structure.
In order to achieve the object of the present invention, we provide a variable valve lift device, comprising: a tappet case which makes contact with one of cams arranged at a camshaft driven rotationally due to a crankshaft of an internal combustion engine and which is driven reciprocally due to the rotation of the cam; a sway member which is so supported in the tappet case as to allow sway of the sway member and which has a sliding face displacing a valve stem in an axial direction of the valve stem; and a sliding member which is so arranged in the tappet case as to allow sliding of the sliding member and which sways the sway member when the sliding member is slid. In this way, since it is possible to simplify the internal structure of the tappet case, the VVL device can be reduced in size and weight. Therefore, it is possible to make full use of the reduction of inertial mass defined as the maximum merit of the direct drive-style VVL device.
With the above arrangement, the cam making contact with the tappet case may have a low-lift cam profile adequate for either one or both of driving condition of middle speed or less and middle load or less of the internal combustion engine. In this way, since the construction is applicable to a camshaft equal to a camshaft used in a normal internal combustion engine without any specialized equipment, the VVL device can be reduced in size and weight. Since the VVL device per se is not operated under the above condition, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, the sway member may be so set as to increase displacement of an intake valve or an exhaust valve with respect to displacement of the tappet case in the axial direction under the driving condition of middle speed or more and middle load or more of the internal combustion engine. In this way, only when the engine is driven under the above condition, it is possible to operate the sway member as a second cam to increase the valve lift. Therefore, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, the sliding member may include a fit hole which allows fit of a reciprocating external piston based on a cylinder head side of the internal combustion engine via a long aperture extending on an outer peripheral face of the tappet case in the axial direction of the tappet case; and a support section supporting a protuberance formed at a position different from a position of a lift section of the sway member which makes contact with an end of the valve stem relative to a center of rotation of the sway member in a state of fitting the external piston in the fit hole. In this way, the tappet case moves toward the valve in accordance with the cam-profile when the external piston is fitted in the fit hole of the sliding member, whereas the movement of the sliding member in the axial direction is restricted. Since the sway member moving together with the tappet case sways due to the support section, the contact section of the sway member slides on the valve stem. Therefore, it is possible to displace largely the valve stem toward the valve side in accordance with the profile of the contact section of the sway member.
With the above arrangement, a sliding face of the sliding member making contact with the valve stem may have an advantageous shape for exhibiting the same abrasion and sliding resistance properties as the cam profile. In this way, since it is possible to prevent the occurrence of an abrasion and a pinch of the sway member, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, the sway member may have a sliding face having a shape adequate for locking the sliding member due to a load derived from the valve stem in a state of not fitting the external piston in the fit hole of the sliding member. In this way, since it is possible to prevent the occurrence of an abrasion and a pinch of the sway member, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, two or more symmetrical sway members may be arranged in one tappet case. In this way, since it is possible to reduce the pinch occurred between the sway member and the tappet case, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, the sway member may be made of high-strength sintered materials or materials for exhibiting abrasion and sliding resistance properties. In this way, since it is possible to increase the durability of the VVL device and to ensure the sliding resistance property between movable parts with stability over a long term, it is possible to increase the operational reliability of the VVL device.
With the above arrangement, the external piston may move forward to fit in the fit hole of the sliding member due to application of hydrodynamic pressure or electromagnetic force and may move back in a direction of unlocking the fit relation due to a mechanical biasing force when the hydrodynamic pressure or electromagnetic force is not applied. In this way, since the external piston is fitted detachably in the fit hole, it is possible to increase the reliability of operation of the VVL device.
With the above arrangement, the external piston may restrict rotation of the tappet case and the sliding member in a peripheral direction thereof and displacement in the axial direction when the external piston is fitted in the fit hole of the sliding member and restricts the rotation of the tappet case and the sliding member in the peripheral direction thereof when the fit relation is unlocked. In this way, since it is possible to locate the tappet case and the sliding member in the peripheral direction thereof at all times, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, the long aperture of the tappet case may have a width slightly larger than an outer diameter of the external piston. In this way, since the external piston is fitted in the long aperture to restrict the rotation of the tappet case and to locate the tappet case in the peripheral direction, it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, a central axis of swaying of the sway member may be parallel to the camshaft. In this way, it is possible to reduce a pinch occurred between the sway member and the tappet case and to increase the durability and the reliability of the VVL device.
With the above arrangement, two external pistons facing to each other may be so arranged reciprocally as to be symmetrical about a central axis of one tappet case. In this way, a pinch occurred between the sliding member and the tappet case can be reduced and it is possible to increase the durability and the reliability of the VVL device.
With the above arrangement, the external piston may be made of high-strength sintered materials or materials for exhibiting abrasion and sliding resistance properties. In this way, since it is possible to increase the durability of the VVL device and to ensure the sliding resistance property between movable parts with stability over a long term, it is possible to increase the operational reliability of the VVL device.
With the above arrangement, the sliding member may be a ring-shaped member, which is so arranged in the tappet case as to slide between an uppermost section of the tappet case and a ring-shaped stopper fixed in the tappet case. In this way, the sway member making contact with the valve stem can be arranged in the tappet case and the sliding member. Therefore, since the respective parts can be arranged in such a compact space, the VVL device per se can be reduced in size and weight.
With the above arrangement, it may further comprise a drain hole arranged in the fit hole of the sliding member. In this way, it is possible to improve responsibility of the external piston.
With the above arrangement, it may further comprise a tapered section which has a width in the axial direction which is larger than a clearance defined between a base circle of the cam and an upper section of the tappet case and which is formed at an opening of the fit hole of the sliding member. In this way, even if the displacement of the sliding member in the axial direction with respect to the external piston is comparable to the clearance above or so, it is possible to lead the external piston to the fit hole of the sliding member due to the tapered section. Therefore, it is possible to ensure the quick fitting of the external piston in the fit hole of the sliding member with reliability and to improve the responsibility and the operational reliability.
With the above arrangement, the sliding member may be made of high-strength sintered materials or materials for exhibiting abrasion and sliding resistance properties. In this way, since it is possible to increase the durability of the VVL device and to ensure the sliding resistance property between movable parts with stability over a long term, it is possible to increase the operational reliability of the VVL device.
With the above arrangement, it may further comprise a recess arranged at an upper section of the tappet case to allow arrangement of a clearance adjustment member adjusting a clearance defined between a base circle of the cam and an upper section of the tappet case, and may further comprise a bearing arranged integrally in the tappet case to support a sway axis of the sway member. In this way, when the internal combustion engine is assembled and fabricated, the clearance adjustment member having a thickness corresponding to a measured value of the clearance can be arranged as appropriate. Since the bearing for the sway axis of the sway member is arranged integrally in the tappet case, it is possible to reduce a component count to make an assembling work more efficient and to reduce the cost of parts.
With the above arrangement, it may further comprise a disk-shaped clearance adjustment member arranged between the sway member and the valve stem. In this way, it is possible to enlarge an area of abutment against the sway member and to adjust the clearance defined between the base circle of the cam and the upper section of the tappet case.